This invention relates to reinforced composites containing photopolymerised and thermosettable compositions and fibrous reinforcing materials, and to methods for their production.
Composite structures are commonly made by impregnating fibrous materials, such as fibres of paper, glass, aromatic polyamide, or carbon, metal filaments, and whiskers, usually in the form of layers, with a solution, in a volatile solvent, of a solid thermosettable resin and a heat-activated curing agent for the resin, causing the resin to solidify by evaporation of the solvent, and, when desired, curing the resin composition in the resultant so-called "prepreg" by the action of heat. Frequently, the prepregs are stacked before heat-curing, so that a multilayer laminate is formed.
Sheet moulding compounds are made by impregnating chopped fibrous reinforcement with a liquid thermosettable resin, and then causing the resin to thicken by chemical means such as by reaction of carboxyl groups in the resin with magnesium oxide. The resin is subsequently heat-cured.
Composite structures may also be prepared from films of a thermosettable resin composition by laying a film of the composition containing the resin and the curing agent on a fibrous reinforcement and applying heat and pressure so that the resin composition flows about the fibres but remains curable, and then heating further when desired so that the resin composition is cured by the heat-activated curing agent.
These methods suffer from certain drawbacks. Some thermosettable resin compositions are solid, and if a solvent is used to dissolve the components of the composition it is not always possible to eliminate all traces of it before the final curing takes place, and in consequence the cured composite may contain voids caused by evaporation of such residual solvent. Solvents may cause difficulties due to their toxicity or inflammability or to pollution. If a film adhesive is used, it is usually cast from a liquid thermosettable resin and this is then advanced to the solid state; such a process adds considerably to the cost of the composite. The methods also require a considerable expenditure of heat energy, either to evaporate the solvent or to advance the resin.
In recent years, thermosetting resins containing maleimide or similar groups have become of interest; however, they sometimes present difficulties in the production of composite structures from them, since they, and preferred curing agents therefor, are often solids and cannot readily be thickened for the production of sheet moulding compounds. These thermosetting resins have to be applied to the fibrous reinforcement from solution in an organic solvent, such as N,N-dimethylformamide or dimethyl sulphoxide, with the disadvantages just mentioned. Further, they have not hitherto been readily obtainable in film form.